function obj = plus(varargin)
    
    if nargin == 1
        % sum up all data objects.
        
    end
    
    
    if nargin == 2
        a = varargin{1};
        b = varargin{2};
        if isa(a,'sp_data') && isa(b,'sp_data')
            
        end
        
        if isa(a,'sp_data') && isnumeric(b)
            disp('Hello')
            obj = sp_data(a);
            set(obj,'y',get(a,'y') + b);
        end
        
        if isa(b,'sp_data') && isnumeric(a)
            obj = sp_data(b);
            set(obj,'y',get(b,'y') + a);
        end
    end
        
    